I think the problem lies in how those who hold such a high opinion of themselves, study God's Word. They always have a verse or a passage of Scripture upon which they base their claim. However, I have found that when studying God's Word, one needs to take all that the Bible says on a specific subject, instead of building a doctrine on one or two verses. For example: what does God's Word say about divorce? The Bible says God hates divorce (Mal. 2:16; Mk. 10:11-12; etc.), and yet, the Old Testament Law allowed it (Deut. 24:1; Mk. 10:4; etc.). The apparent contradiction can easily be explained when we understand why God accepted the fact that people divorce; there comes a time in some marriages when one or both of the couple's love for their partner has turned to hate. Jesus said it this way: "...Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so" (Mt. 19:8)! So you can see that understanding an issue from God's perspective, requires one to consider ALL that the Word says on a subject.
Another problem people have is that they are unfamiliar with the original languages of the Bible. They have chosen a Bible version in their language, and consider it to be like Jesus: perfect! Most English speaking believers, have in the past, considered the King James Version to be 100% accurate. But my question to them is: Which KJV? There have been several "upgrades" over the 400 plus years since the first edition in A.D. 1611. Another question I have asked in the past, is: If the 1611 version is what all of the others are based upon, then why don't they all have the Apocrypha in their Bibles? The 1611 version did! Somehow, I have, so far, been able to avoid being burned at the stake, but not because some have not wanted to.
That being said, let's look at a couple of verses upon which our "perfect Christians" base their view. In 1 John 3:6, we read "Whosoever abideth in Him sinneth not: whosoever sinneth hath not seen Him, neither known Him." And in 1 John 5:18 says: "We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not." In both verses, the word "sinneth" is in the Greek Present, Active, Indicative Case. In other words, one might say that the individual does not currently indulge in habitual sin which characterizes their life - they are not known for their sins, but for their desire to live for Christ! Romans 7:14-25 describes the genuine born again believer as living in a struggle with sin; [note that the verbs are present tense]; Paul wrote:
"For we know that the Law is spiritual: but I am carnal, sold under sin. For that which I do I allow not: for what I would, that do I not; but what I hate, that do I. If then I do that which I would not, I consent unto the Law that it is good. Now then it is no more I that do it, but sin that dwelleth in me. For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not. For the good that I would I do not: but the evil which I would not, that I do. Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me. I find then a law, that, when I would do good, evil is present with me. For I delight in the Law of God after the inward man: But I see another law in my members, warring against the Law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! Who shall deliver me from the body of this death? I thank God through Jesus Christ our Lord. So then with the mind I myself serve the Law of God; but with the flesh the law of sin."
I seriously doubt that any student of the Bible would consider the Apostle Paul and unbeliever because he sinned. If that were the case, half of the New Testament would have been written by a man who cannot be trusted! In the same letter that the above "sinneth not" verses appear, the Apostle John wrote in 1 John 1:8-10:
"If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His Word is not in us."
One of the greatest hindrances to the Christian's evangelical efforts, is that the world also believes Christians are supposed to be perfect! We are not! But the One about Whom we speak, certainly is!